Prolonging the shelf life of fresh root vegetables

ABSTRACT

The shelf life of root crops such as raw carrots can be substantially increased by subjecting the raw carrots to a process including (a) a mild heat treatment effective to reduce the microflora of the carrots but not to adversely affect the organoleptic qualities of the raw carrots, (b) the rapid cooling of the heat treated vegetable and placing the vegetable in a sealed container to prevent microbial recontamination and maintain the vegetable in a viable condition.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a process for prolonging the shelf life offresh vegetables. More particularly, it relates to prolonging the shelflife of fresh root crops. This invention especially relates toprolonging the shelf life of carrots by a process which includes a hotwater treatment, a cooling step and a packaging operation.

2. Description of the Prior Art

Fresh fruits and vegetables are extremely perishable commodities.Heretofore many techniques have been employed to protect such foodproducts from oxidative degradation, mold attack and moisturepenetration and to preserve the freshness, texture and color of thefresh produce. One of the earliest means of lengthening the shelf lifeof fruits and vegetables was refrigeration. However, most fresh producewhen stored under reduced temperatures for prolonged periods showsadverse effects on the taste, odor or quality of the product frommicrobial and mold growth above 35° F. In addition, storage temperaturesbelow 35° F. often show chill injury to the tissue of the produce.Hence, in many instances refrigeration alone is not effective inachieving the desired shelf life for a particular fruit or vegetable.

Coating fresh fruits and/or vegetables is another of these techniqueswhich has been employed with varying degrees of success. Not only mustthe coating be effective in prolonging the useful shelf life of thefresh product, but the appearance of the commodity must not be alteredfrom that of its natural state. At a minimum, this natural appearancemust not only remain unchanged but should ideally be enhanced especiallywhen the fruit or vegetable will be displayed for sale. The selection ofa coating material is further complicated where the fruit or vegetableis to be consumed in its natural state and it is considered essentialthat there be no need to remove the coating. In that event, the coatingmaterial must not only be edible, it must not affect or alter thenatural organoleptic characteristics of the fresh fruit or vegetable.

Typical of these prior art coatings are the wax emulsions of U.S. Pat.Nos. 2,560,820 of Recker and 2,703,760 of Cunning. Coatings of naturalmaterials have been employed including milk whey (U.S. Pat. No.2,282,801 of Musher), lecithin (U.S. Pat. Nos. 2,470,281 of Allinghamand 3,451,826 of Mulder), gelatin together with polyhydric alcohol (U.S.Pat. No. 3,556,814 of Whitman et al.) and protein (U.S. Pat. No.4,344,971 of Garbutt). Polymers have also been used extensively, viz., athermoplastic polymer (U.S. Pat. No. 2,213,557 of Tisdale et al.), vinylacetate polymer (U.S. Pat. No. 3,410,696 of Rosenfield), a hydrophilicpolymer (U.S. Pat. No. 3,669,691 of De Long et al.) and the combinationof a water soluble polymer and a hydrophobic material (U.S. Pat. No.3,997,674 of Ukai et al.). Cellulostic materials have found utility incoating fruits and vegetables including hydrated cellulose (U.S. Pat.No. 1,774,866 of Beadle), a combination of cellulose and wax (U.S. Pat.No. 2,364,614 of Beatty), cellulose ether in combination with a fattyacid ester (U.S. Pat. No. 3,471,303 of Hamdy et al.) or monoglycerideand a fatty acid metal salt (U.S. Pat. No. 3,461,304 of Hamdy et al.),or a sucrose ester of a fatty acid (U.S. Pat. No. 4,338,342 of Tan etal.)

Food preservation has for many years employed such mutually exclusiveprocesses as dehydration and freezing. Both of these operations ofteninclude a heat treatment, known as blanching, which is conducted priorto the dehydration or freezing step. Blanching is said to reduce enzymeor bacteria level and to prevent or minimize undesirable changes duringstorage in the dry or frozen state, such as changes in color, odor ortexture or loss of vitamins. Blanching may be conducted with steam(e.g., U.S. Pat. No. 2,373,521 of Wigelsworth), hot water (U.S. Pat. No.2,515,025 of Vahl et al.), hot oxygen-free gas (U.S. Pat. No. 3,801,715of Smith et al.) or hot air (U.S. Pat. No. 3,973,047 of Linaberry etal.)

U.S. Pat. No. 2,780,551 of Guadagni is also concerned with thepreservation of raw fruits or vegetables by freezing. A heatpretreatment is employed here also, but it is a mild heat treatment tocause partial enzyme inactivation. The mild heat treatment may beconducted by immersing the food in a liquid which is heated to atemperature in the range of 150° to 212° F. for a period of from about10 seconds at the higher temperatures to about 10 minutes at the lowertemperatures. The heat treatment is described as heating the food at atemperature and for a period of time sufficient to inactivate theenzymes in the surface layers of the food but not the enyzmes throughoutthe food and not sufficient to sterilize or cook the food. Thecombination of mild heat treatment and freezing produces a product whichwill maintain its fresh color, taste and appearance after prolongedstorage.

U.S. Pat. No. 2,619,424 of Masure discloses a process for preparingdehydrated carrots having improved storage qualities. The carrots aredehydrated, then moistened with water, optionally containing starch, andthen the carrots are dehydrated to a moisture level below about 10%. Theuse of starch assists in preserving the color of the final productalthough the absorbed water is the primary factor in improving thestability of the color and the carotene content.

It is an object of this invention to prolong the post harvest life ofthe fresh root crops.

It is another object of this invention to improve the shelf life offresh carrots.

It is a further object of this invention to provide raw carrots in aready-to-eat form and to improve the shelf life of these fresh carrots.

SUMMARY OF THE INVENTION

These and other objects are achieved by subjecting the raw rootvegetable to a mild heat treatment and a cooling treatment and thensealing the vegetable in a container.

In particular, this invention concerns a process of maintaining a freshroot vegetable in a viable form which comprises:

(a) subjecting a fresh root vegetable to elevated temperature and timeconditions effective to reduce the microflora concentration in colonyforming units (CFU)/g greater than 10² while substantially maintainingthe sensory qualities of color, taste and texture of the fresh rootvegetable,

(b) rapidly cooling the heat treated vegetable to below about 25° C. inless than about two minutes, and

(c) providing the vegetables in a sealed container effective to preventmicrobial recontamination of the vegetable and having a gas permeabilityeffective to maintain the vegetable in a viable condition.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention relates to a process of improving the useful shelflife of fresh vegetables, particularly root vegetables, especiallycarrots. The invention also relates to the vegetable produced by thisprocess. In one embodiment this invention relates to carrots prepared asa ready-to-eat raw snack food and provided in a package which may bestored for a period substantially longer than raw fresh carrots whichhave not been treated by the process of the present invention. Carrotsprovided in this fashion are ideal for use with party dips or forinclusion in a lunch box or a picnic basket since they require nopreparation and can be served or eaten "right out of the package."

The basic purpose of the process of this invention is to reduce theendogenous microflora present in a raw fresh root vegetable whilemaintaining the vegetable in a viable form and without undue damage tothe vegetable cell tissue so that when it is subsequently eaten, it willprovide substantially the same sensory qualities of appearance, odor andtaste as the freshly harvested raw vegetable. Without treatment inaccordance with this invention, the growth of microflora in the freshvegetable will in a short time adversely affect the organolepticqualities of the vegetable even if stored under refrigeration. Bypracticing the process of the present invention, the useful shelf lifeof fresh carrots, for example, can be prolonged by about 14 to about 21days at a 10° C. storage temperature.

The vegetables which are particularly adapted to the process of thepresent invention are those which are known collectively as rootvegetables or root crops, particularly those which are eaten in the rawstate. These vegetables include carrots, turnips, rutabaga and radishes.Carrots are especially well suited to being treated in accordance withthe present invention which will be explained below using carrots as thevegetable being processed. This is done for illustration purposes andalthough carrots are especially preferred when practicing thisinvention, other root crops, as explained above, may be employed.

In accordance with the present invention, the carrots are subjected to amild heat treatment to substantially reduce the microflora content thenare rapidly cooled and finally are placed in a sealed container.

In most instances the carrots will be provided in a condition wherebythe consumer can eat the carrots without any preparation. Thus, beforethe carrots are shipped to the processing plant to be treated inaccordance with the present invention, the harvested carrots should betopped to remove the green leafy top and washed to remove the soil fromthe exterior surface of the carrots. This can be performed in the fieldduring harvesting or just prior to shipping to the processing andpackaging plant. The preparation continues at the plant with peeling ofthe carrots which may be achieved by any of several known methods:

(a) steam peeling using a batch type peeler such as an Odenberg peeler,K & K Model 100 or an FMC continuous peeler,

(b) lye peeling usino a peeler such as an A. K. Robins ferris wheel typeemploying a lye concentration of 5-20%, or

(c) mechanical peeling using an interchangeable roller type peeler suchas a Magnason Model HL. The peels can be removed from the carrots byemploying equipment such as an A. K. Robins drum washer or a MagnasonMagnu washer with stud rubber and brush rolls.

Since the carrots are to be provided in a ready-to-eat form, the carrotsshould be of a small diameter variety which are cut to the desiredlength, usually 3-5 inches. For larger diameter carrots, they should becut to provide carrot sticks of about 3-5 inches in length and a crosssection of about 1/4-1/2 inch by 1/4-1/2 inch.

In the practice of this invention, the initial step is a mild heattreatment. The mild heat treatment must be conducted at a temperatureand for a period of time which are effective to reduce the endogenousmicroflora without causing undue damage to carrot cell tissue whichcould affect the sensory qualities of a fresh, raw carrot. Excessiveheat treatment causes the carrots to have a cooked appearance and tasteand to suffer a loss of turgor, i.e., the ability to hold water. Incontradistinction, insufficient heat treatment results in no appreciableand consistent reduction of microflora. To be effective, the treatmentmust cause a reduction of microflora count of at least 10², preferablyat least 10⁴. It is possible when practicing the heat treatment of thisinvention to reduce the microflora level to below about 3 colony formingunits (CFU) per gram of carrots. In one embodiment of the heattreatment, the carrots are immersed for about 30 to about 180 seconds inwater maintained at about 45 to about 55° C. Those skilled in the artcan determine the optimum conditions for particular carrots without anundue amount of experimentation by evaluating the microflora countbefore and after a series of heat treatment screening tests. Not onlymust the heat treatment cause the required reduction in microflora, butthe carrots must maintain the organoleptic qualities of odor,appearance, taste and mouth feel evidenced by freshly harvested rawcarrots.

The next step involves a rapid cooling of the heat treated carrots. Therapid cooling is necessary to minimize surface cell heat destruction andreduce enzymatic activity which adversely affect color, odor, taste andtexture. As the carrots are removed from the heat treating step, theyare rapidly cooled to below about 25° C., preferably below about 15° C.in liss than about 2 minutes. This cooling may be accomplished invarious ways, including spraying the carrots with cold water, immersingthem in cool water, passing cold air over the carrots, or the like.Other cooling methods known in the art may also be used instead of or inconjunction with any of these cooling methods. In one preferredembodiment, the rapid cooling is achieved by immersing the carrots incold water, preferably chlorinated (10-15 ppm). In another preferredembodiment the carrots are subjected to vacuum cooling to achieve therequired rapid cooling.

The heat treated and cooled carrots are now ready for the finaloperation of packaging. The carrots are placed in a sealed package toprevent undue moisture loss as measured by texture and to preventmicrobial recontamination. The packaging material should have a gaspermeability effective to permit sufficient respiration for maintainingthe carrot tissue in a viable condition. Typically, useful packagingfilms should have a permeability of about 75 to about 200 cc of O₂ /100in² -atm.-24 hrs. and about 400 to about 600 cc of CO₂ /100 in² -atm.-24hrs. and a moisture transmission rate of less than 1.0 g/100 in²-atm.-24 hrs. Further, the ratio of product mass to package surfaceshould be in the range of about 1 to about 6 g/in². Packages can be madefrom flexible or semi-rigid materials in various shapes and formsincluding three sided pouches and thermoformed tubs to provide anattractive, eye-catching package. Foamed polypropylene has been found tobe useful packaging material in the practice of this invention.

For maximum shelf life, the packaged carrots should be maintained underrefrigeration at a temperature of above 2° to below 15° C., preferablyabout 2° to about 4° C. Packaged carrots prepared in a-cordance with theprocess of this invention have a shelf life of about 14 to about 21 dayslonger than packaged carrots which have not been similarly processed.

Although, the heat treating, cooling and packaging of the carrots inaccordance with this invention significantly improves the shelf life ofthe fresh carrots, several other optional steps may be included in theprocess of this invention to further enhance the shelf life and/or theappearance of the carrots.

Thus, following the rapid cooling of the carrots by any of the methodsdescribed above, the carrots are preferably subjected to a vacuum in therange of about 1 to about 12 inches Hg absolute for about two to aboutten minutes in the presence or absence of water, which may optionally bechlorinated. The vacuum should be broken in the presence of water,preferably chlorinated. This may be accomplished by having the carrotsimmersed in water or by spraying water over the carrots when the vacuumis broken. Ideally, this use of vacuum can be accomplished inconjunction with a vacuum cooling which is one of the preferred coolingtechniques. Breaking the vacuum in the presence of water significantlyimproves the color of the carrot pieces in that the color looks brighterand deeper as compared to carrot pieces not so treated. This is thoughtto be caused by the removal of inter and intra cellular gases andreplacing them with water. Employing chlorinated water here will providean additional benefit in that the initial microflora will be furtherreduced, further prolonging shelf life. Breaking the vacuum withchlorinated water when used in combination with the hot water heattreatment provides a synergestic improvement.

Since the cooled carrots may contain surface water after they are cooledand, optionally, subjected to vacuum conditions, this water should beremoved before the carrots are further processed. Any of several knowntechniques can be employed, such as, centrifuging or contacting with dryair at temperatures of about 10 to about 20° C. or the like.

In another optional embodiment the heat treated and cooled carrots maybe provided with a coating before they are placed in the sealedprotective package. This coating controls the state of water at thecarrot's surface. It also acts as a partial barrier to oxygen and carbondioxide exchange thus further slowing biochemical reactions andprolonging shelf life. Since, ideally, the coating will be consumed whenthe carrots are eaten, the coating material selected should not only beedible, it must not adversely affect the natural taste of raw carrots.Further, the coating material should enhance the surface appearance ofthe carrots when they are displayed for sale. Previous studies havedetermined that microorganisms do not grow in an environment ofrestricted water availability and some consider that the partialpressure of the water vapor, in equilibrium with the growth media, suchas a food product, is the criterion of the water availability. It nowappears that this criterion is the thermodynamic state of the waterrather than the equilibrium relative humidity. More specifically, therelative humidity is related to the average fugacity or escapingtendency of water. Bacteria, on the other hand, grow only in thepresence of water which is associated with other water molecules, i.e.,clustered water, rather than in the presence of water associated with asingle polar bonding site on macromolecules, i.e., bound water.Macromolecules such as hydrocolloids are ideally suited for use as acoating material, provided they meet the organoleptic standards as well.Hydrocolloids such as microcrystalline cellulose or modified tapiocastarch are particularly preferred. Typically coatings of these materialsconstitute about 0.05 to about 1.0 of the weight of the carrot. Thehydrocolloids must be effective in regulating the availability of waterat the surface of the carrots. The hydrocolloids can also slow down thepermeation of oxygen from the environment into the carrot tissue and thepermeation of carbon dioxide from the tissue into the surroundingenvironment.

The following examples illustrate the practice of the invention.

EXAMPLE I

The effect of hot water treatment on the initial aerobic microbial countof fresh carrots was evaluated.

A number of fresh carrots were washed, peeled and cut into 3"×3/8"×3/8"sticks. The carrot sticks were subjected to various hot watertreatments, and then the initial aerobic microbial count of the treatedcarrots in terms of colony forming units (CFU) per gram was measured.

The various treatments and the results are shown in Table I below. Thesedata indicate that the appearance and taste of fresh carrots can besubstantially retained when subjecting the carrots to hot watertreatment while significantly reducing the surface microbial count.

                                      TABLE I                                     __________________________________________________________________________    The Effect of Hot Water Treatment on                                          Initial Aerobic Microbial Count of Fresh Carrots                              Description   Initial C.F.U./gram                                                                     Initial Observation                                   __________________________________________________________________________    Fresh Prepared Carrots                                                                      3.0 × 10.sup.4                                                                    Semi-dry surface, good flavor                         No Hot Water Treatment  Medium-high rigidity, light                                                   orange color                                          Agitated Water @ 45° C.                                                              1.0 × 10.sup.3                                                                    Medium rigidity, good flavor                          Immersion time: 1 minute                                                                              and texture, slightly moist                                                   surface                                               Agitated Water @ 45° C.                                                              3.0 × 10.sup.2                                                                    Medium rigidity, slight                               Immersion time: 3 minutes                                                                             flavor loss, moist surface,                                                   color not as intense as with                                                  higher temperature                                    Agitated Water @ 55° C.                                                              <3        Medium rigidity, good carrot                          Immersion time: 1 minute                                                                              flavor, slightly moist orange                                                 surface                                               Agitated Water @ 55° C.                                                              <3        Medium to soft rigidity, slight                       Immersion time: 3 minutes                                                                             cooked taste, some flavor loss,                                               orange moist surface                                  Agitated Water @ 60° C.                                                              <3        Soft, rubbery texture, cooked                         Immersion time: 3 minutes                                                                             flavor, good orange color                             Agitated Water @ 65° C.                                                              <3        Medium rigidity, slight cooked                        Immersion time: 1 minute                                                                              flavor, orange moist surface                          Agitated Water @ 65° C.                                                              <3        Soft, rubbery texture, cooked                         Immersion time: 3 minutes                                                                             flavor, orange moist surface                          Agitated Water @ 70° C.                                                              <3        Medium rigidity, moist orange                         Immersion time: 0.50 minutes                                                                          surface, slight flavor loss                           Agitated Water @ 70° C.                                                              <3        Medium to soft rigidity, moist                        Immersion time: 0.75 minutes                                                                          orange surface, flavor loss                                                   more extensive                                        __________________________________________________________________________

EXAMPLE II

The effect of hydrophilic polymer on the shelf-stability of hot watertreated carrots was evaluated.

A number of carrots were prepared in stick form as in Example I and werethen subjected to a series of hot water treatments and polymer coatings.After the treated carrots were stored for 13 days, the microbial countof each sample was measured.

A description of the various treatments and the results of theevaluation are shown in Table II below. These data indicate that acoating of a hydrophilic polymer can significantly improve the storagestability of flesh carrots which have been treated with hot water toreduce the microflora content of the carrots.

                  TABLE II                                                        ______________________________________                                        The Effect of Hydrophilic Polymers on                                         The Shelf-Stability of Hot Water Treated Fresh Carrots                                           Colony Forming Units                                       Description        per gram After Storage                                     ______________________________________                                        Control            .sup. >4.0 × 10.sup.10                               Agitated Water @ 55° C.                                                Immersion time: 2.5 minutes                                                   Agitated Water @ 55° C.                                                                    2.2 × 10.sup.8                                      Immersion time: 2.5 minutes                                                   Polymer/Usage: 0.05% Micro-                                                   crystalline Cellulose                                                         Agitated Water @ 55° C.                                                                    1.4 × 10.sup.9                                      Immersion time: 2.5 minutes                                                   Polymer/Usage: 0.1% Micro-                                                    crystalline Cellulose                                                         Agitated Water @ 55° C.                                                                    1.0 × 10.sup.8                                      Immersion time: 2.5 minutes                                                   Polymer/Usage: 0.2% Micro-                                                    crystalline Cellulose                                                         Agitated Water @ 55° C.                                                                   <2.0 × 10.sup.9                                      Immersion time: 2.5 minutes                                                   Polymer/Usage: 0.2% Tapioca                                                   Starch                                                                        ______________________________________                                    

EXAMPLE III

The effect of hot water treatment and vacuum on the storage stability ofcarrots was evaluated.

A number of fresh carrots were prepared in stick form as in Example Iand were then subjected to various combinations of hot water treatmentand vacuum. The microbial count of the treated carrots was measuredimmediately after the various treatments and again after prolongedstorage which ranged from 15 to 27 days.

A description of the various treatments and the results of theevaluation are presented in Table III These data indicate that thecombination of hot water treatment and vacuum have a synergistic effectin controlling microbial growth in fresh carrots.

                  TABLE III                                                       ______________________________________                                        The Synergistic Affect of Hot Water Treatment                                 and Vacuum in Controlling Microbial Growth                                                             Storage                                                            Initial    Time     C.P.U./gram                                 Description   C.F.U./gram                                                                              (Days)   <10.sup.13                                  ______________________________________                                        Fresh prepared carrots                                                                      3.7 × 10.sup.4                                                                     15    days 5.8 × 10.sup.13                     Chlorinated Water                                                                             3 × 10.sup.3                                                                     15         2.5 × 10.sup.13                     soak 5 minutes                                                                (50-100 ppm)                                                                  Chlorinated Water                                                                           1.6 × 10.sup.3                                                                     17         3.1 × 10.sup.12                     Soak 5 minutes                                                                (50-100 ppm)                                                                  Vacuum 28" Hg for                                                             5 minutes                                                                     Hot Water Treatment                                                                         <3.9 × 10.sup.2                                                                    15         8.2 × 10.sup.15                     55° C. for 2 minutes                                                   Hot Water Treatment                                                                         Neg.       27         8.7 × 10.sup.11                     55° C. for 2 minutes                                                   Vacuum 28" Hg for                                                             5 minutes                                                                     ______________________________________                                    

What is claimed is:
 1. A process of maintaining a fresh root vegetablein viable form which comprises:(a) peeling the surface of a fresh rootvegetable having endogenous microflora present therein, (b) providingthe vegetable in uniform sized pieces, (c) subjecting the vegetablehaving endogenous microflora present therein to elevated temperature andtime conditions effective to reduce the endogenous microfloraconcentration in colony forming units (CFU)/g by a factor greater than102 while maintaining the vegetable in a viable form and substantiallymaintaining the sensory qualities of color, taste and texture of thefresh root vegetable, (d) rapidly cooling the heat treated vegetable tobelow about 25° C. in less than about two minutes, (e) subjecting thevegetable to a vacuum of about 1 to about 12 inches Hg absolute forabout 2 to about 10 minutes, (f) breaking the vacuum in the presence ofwater, (g) removing surface water from the vegetable, and (h) providingthe vegetable in a sealed container effective to prevent microbialrecontamination of the vegetable and having a gas permeability effectiveto maintain the vegetable in a viable condition.
 2. A process accordingto claim 1 including the following steps conducted prior to step(a):removing the green leafy top of the fresh root vegetable, andeffectively washing the exterior surface of the vegetable to removesoil.
 3. A process according to claim 2 wherein the vegetable iscarrots, turnips, rutabaga or radishes.
 4. A process according to claim1 including the following step conducted after step (g) and before step(h):providing a coating of hydrocolloid on the rapidly cooled, heattreated fresh root vegetable, the nature and thickness of the coatingbeing effective to substantially reduce the availability of clusteredwater.
 5. A process according to claim 4 wherein the hydrocolloid ofstep is microcrystalline cellulose or modified tapioca starch and thecoating is about 0.05 to about 1 wt. % of the vegetable weight.
 6. Aprocess according to claim 1 wherein the vegetable is carrots.
 7. Aprocess according to claim 6 wherein step (c) is conducted by contactingthe vegetable with water at a temperature of about 45 to about 55° C.for a period of time of about 30 to about 180 seconds.
 8. A processaccording to claim 6 wherein step (d) is conducted by contacting thevegetable with cold water.
 9. A process according to claim 6 whereinstep (d) is conducted by subjecting the vegetable to vacuum cooling. 10.A process according to claim 1 wherein the vegetable is carrots and step(c) is conducted by contacting the carrots with water at a temperatureof about 45 to about 55° C. for a period of time of about 30 to about180 seconds and step (d) is conducted by subjecting the carrots tovacuum cooling.